1. Field of the Invention
The present invention relates to an improved method of coupling the position data from a resolver to a programmable controller logic processor and to electrical circuitry for achieving such improved coupling.
2. Description of the Prior Art
A typical programmable controller operates on a cycle where input data from input modules is coupled to the logic processor in a processor module of the programmable controller. The input data is input to the user program, the user program is sequentially computed, and then output data is coupled from the processor module to output modules. This input-computer-output process is continuously repeated. The elapsed time from input to output is usually several tens of milliseconds. Also, the processor sequentially reads the data from each input module and sequentially writes the data to each output module. The elapsed time to either read or write eight bits of input or output data from one input module or to one output module is typically about 10 microseconds.
In the prior art, programmable controller systems have been coupled to a resolver where analog signals were generated by the resolver indicating different positions of a machine shaft coupled to the resolver. Typically this was achieved using two modules: one being an analog to digital decoder module and the other module being a coupling module for coupling the digital signals to the controller processor, commonly referred to as a PC handshake module. Very often these two modules were mounted with or within the housing for the resolver located at a position some distance from the programmable controller. As a result, the converted digital signals and the handshake signals were transmitted over a transmission line to the programmable controller logic processor.
The transmission line often times picked-up other signals (static and electromagnetic noise signals) resulted in the adding of noise to the digital signals coming from the PC handshake module and resulted in problems in decoding the digital signals in an input module coupled between the PLC (Program Logic Controller) backplane (I/0 interface) and often times erroneous signals were picked up.
As will be described in greater detail hereinafter, according to the teachings of the present invention, the analog signals from the resolver are coupled by a transmission line directly to a resolver decoder circuit module forming part of a programmable controller system.
The resolver analog signals are fairly strong signals and are not affected by noise picked up by the transmission line between the resolver and the resolver decoder circuit module associated with the programmable controller system. As a result, the resolver decoder circuit module, and more particularly, the electrical circuitry therein, accurately convert the analog signals to digital signals which are transmitted via short transmission lines to the programmable controller logic processor. In this way, the digital signal converted from the resolver analog output signal by the resolver decoder circuitry, is a more accurate and stable signal indicative of machine shaft position.
In other words, according to the teachings of the present invention, the accuracy of a shaft position determination is enhanced by coupling the analog output signals from a resolver directly to a resolver decoder circuit module forming part a programmable controller assembly.
A typical prior art resolver analog-to-digital decoder circuit module provides a sinusoidal excitation frequency to the rotor winding of a resolver. The resolver has two stator windings which are physically located 90.degree. of shaft rotation apart. As the resolver shaft rotates through 360.degree., one stator winding will have its voltage and phase vary to correspond with the sine of the shaft angle, and one stator winding will have its voltage and phase vary to correspond with the cosine of the shaft angle. Resolver analog-to-digital decoder circuitry will couple the excitation frequency and the two stator winding voltages to circuitry that will convert these signals to a binary-weighted representation of the resolver's shaft angle. There also may be circuitry in the resolver analog-to-digital decoder circuitry for converting the binary-weighted shaft position data to BCD coded information. The resolution of the shaft angle is typically one part in 4096 but may be as high as one part in 65,536. Typical shaft rotation speeds range in excess of 1800 RPM.
In some embodiments of a prior art controller system coupled to a resolver, the machine's shaft is directly coupled to a fine resolver while being geared down to a coarse resolver. A typical arrangement would be to have the fine resolver turn 128 times to one turn of the coarse resolver. The resolvers are connected to two resolver analog-to-digital converter circuits as previously described. Because it is difficult to align the two resolvers exactly in a practical arrangement, at the transition points of the coarse resolver output signals there will be a conflict of the readings of the coarse and fine resolver analog-to-digital converter circuits which have a binary-weighted output unless additional logic circuits are employed to provide an unambiguous digital output.
In such an arrangement, the number of the most significant bits of the coarse resolver analog-to-digital converter circuit output corresponding to the gear ratio is an output which has been adjusted according to the relationship of the coarse and fine positions as the most significant bits while the output of the fine resolver analog-to-digital converter is used directly to represent the lower order bits of the shaft position.
In a coarse to fine ratio of 128:1 where the coarse resolver analog-to-digital converter circuit resolution is ten bits and the fine resolution is twelve bits, the binary-weighted position's most significant seven bits will be the adjusted output of the coarse resolver-to-digital converter circuit while the twelve lower order bits will be the output of the fine converter. The binary weighted output, then, can range from 0 to 7FFFF hexadecimal or 524,287. Without using a microprocessor, it is very difficult to first translate such a number to more usable position codes such as 0 to 128,000 Binary Coded Decimal (BCD) and then allow the user to easily offset that position with a decimal number module. The output code will require different circuitry for every gear ratio and every different output code.
Even at moderate shaft speed and typical resolution, multi-byte output data from the resolver analog-to-digital conversion circuitry can be easily updated so that, without synchronization, the data will change from the time that a programmable controller can store the first byte to the time when succeeding bytes can be read, resulting in an output error. As an example, suppose that the resolver analog-to-digital data can vary from 0 to 3599 and that the programmable controller reads first the low eight bits and then the next highest eight bits. Also suppose that when the first eight bits is read, the data is 1999, and the processor correctly reads 99 but before the processor can read the second byte, the data changes to 2000. The processor will then read 20 and the data changes to 2000. The processor will then read 20 and the data that is stored is the incorrect value of 2099. To overcome this problem, a synchronization interface from the resolver analog-to-digital decoder to the programmable controller input modules is either incorporated into the resolver analog-to-digital decoder or added as an external module.
Synchronization interface circuitry can be provided to receive a digital transition from a programmable controller output module at its input and then freeze the value of the resolver analog-to-digital decoder's output data after a fixed amount of time, thereby enabling the programmable controller to read the correct data value.
One difficulty with this approach is the additional delay and programming overhead of having the programmable controller first output data and then read the input data. Another difficulty is that additional modules external to the programmable controller are required to convert the resolver shaft position to digital position and secondly, to synchronize the data to the programmable controller's input/output scan.
In addition, the conventional input modules necessary to read the data into the programmable controller are, in themselves, expensive.
Also, with rapidly changing input position data, from the time that an external synchronization circuit freezes the position data until such time that the programmable controller can input the data, the position may have changed so much that computed decisions based on the position data may have little value.
As will be described in greater detail hereinafter, according to the teachings of the present invention a resolver decoder input module is provided which includes a resolver, synchronization logic circuitry for electrically aligning the digital data bits converted from an analog coarse resolver signal with the digital data bits converted from an analog fine resolver signal to ensure correct digital data at transitions from numbers such as 1999 to 2000.
Further, as will be described in greater detail hereinafter a user entry mechanism such as thumbwheel switches and PROM conversion circuitry are provided in the resolver decoder input module of the present invention for enabling entry of a binary coded decimal offset and conversion of same to a digital signal for combining with the converted resolver digital signal.
Such PROM conversion circuitry is not affected by the scale factor, has a scale factor incorporated therein and can accommodate different scale factors.
Also, PC handshake circuitry is provided in the resolver decoder input module of the present invention in the form of position change lockout circuitry.